In certain metallurgical and chemical processes in which fluoride-containing raw materials are treated, HF is obtained as a by-product which, because of environmental regulations, must be controlled so as not to pollute the immediate environment.
For example, during the defluorination of phosphate rock, as practiced in the phosphate industry, HF gas is evolved. Because of environmental regulations, the gas from defluorination reactors is thoroughly scrubbed to remove the HF before its discharge into the atmosphere. Normally, water is used for scrubbing which results in a dilute hydrofluoric acid solution. The strength of the hydrofluoric acid produced generally depends on the type of scrubbing system used. In current practice, such waste solutions may contain from 1 to 8 percent HF. The concentration could be conceivably higher by employing several stages in a countercurrent system. Obviously, as environmental regulations regarding fluoride emissions become more stringent, disposal of the acid becomes more critical. While one method is to simply neutralize the HF and then throw it away, this procedure is expensive and undesirable. The liquor is too dilute and contaminated with impurities such as silicon and phosphorus to market as commercial hydrofluoric acid. Thus, it would be desirable to develop a process to recover the fluoride as a marketable compound, thereby utilizing a cheap source of fluoride while solving disposal problems.
A fluoride compound which may be produced from dilute hydrofluoric acid is potassium hexafluotitanate (K.sub.2 TiF.sub.6), a chemical which may be employed as an addition agent for molten baths of aluminum in which the titanium serves as a grain refining agent.
For this particular application, K.sub.2 TiF.sub.6 should meet specifications for certain chemical and physical characteristics. It is important that the product be dry and granular for safe and easy addition to a molten bath. As discussed herein, K.sub.2 TiF.sub.6 can be produced either as hydrated crystals or as anhydrous crystals. The hydrated crystals can be dried to-remove the chemically combined water and to obtain the anhydrous form. However, this process generates a very fine product which may cause dusting problems. Coarse anhydrous crystals are preferred in that they are denser and stronger than the hydrated form. It is important to keep the impurity contents such as TiO.sub.2, iron, and silicon at very low levels.
It should be noted that several processes have been patented in the past for the manufacture of K.sub.2 TiF.sub.6 but few have been commercialized due to the requirements for expensive reagents or severe operating conditions. One such process, disclosed in U.S. Pat. No. 2,717,197, suggests reacting potassium fluoride dissolved in commercial hydrofluoric acid with titanium tetrachloride at temperatures up to 70.degree. C., and cooling to room temperature to crystallize K.sub.2 TiF.sub.6. The product obtained was "finely crystalline". In another process disclosed in U.S. Pat. No. 2,694,616, a TiO.sub.2 source is mixed with a potassium salt and coke and calcined at 1800.degree. F. The sintered calcine is crushed to -200 mesh and then reacted with liquor containing KCl and CaF.sub.2. Sulfuric acid is next added to lower the pH to 3-5, and the slurry is maintained at boiling as calcium sulfate is precipitated. After removal of CaSO.sub.4, the filtrate is concentrated by evaporation, and the K.sub.2 TiF.sub.6 then crystallized. Other processes involve reacting titanium sulfate with fluoride solutions containing KF or CaF.sub.2, as described in U.S. Pat. No. 2,475,287.
Present commercial practice for the production of potassium hexafluotitanate (U.S. Pat. No. 2,568,341) involves the dissolution of a titaniferous source, generally ilmenite, in concentrated hydrofluoric acid. Ferric ions in solution after the digestion stage are reduced by the addition of scrap iron, so that precipitation of insoluble K.sub.3 FeF.sub.6 may be avoided. The titanium and iron-bearing fluoride solution is separated from the insolubles. After heating the liquor to at least 70.degree. C., a potassium salt, preferably chloride or nitrate, is added. At some point prior to crystallization, sulfuric acid, in an amount ranging from 5 to 50 percent of the HF used, is added to promote formation of large crystals. The liquor is cooled to ambient temperature to produce crystals of K.sub.2 TiF.sub.6. The crystals are filtered, washed with cold water, and dried at about 100.degree. C. in a conventional drying oven.
The main shortcomings of the existing commercial practice is the need to use concentrated hydrofluoric acid and the use of sulfuric acid to aid crystallization. Although both acids are available commercially in concentrated form, they are expensive and add considerably to the cost of the K.sub.2 TiF.sub.6 production. The process of the present invention overcomes these shortcomings. The hydrofluoric acid source may be a dilute solution or a waste stream of dilute HF from the phosphate industry. Large anhydrous crystals can be obtained by either aging the hydrated crystals in the dilute hydrofluoric acid or by providing anhydrous K.sub.2 TiF.sub.6 crystals as seed, thus eliminating the need for sulfuric acid.